Isobaric chemical tags are tools used in mass spectrometry (MS)-based quantitative proteomics for labeling analytes in different samples to be quantified concurrently. Isobaric tags are usually a set of molecules with identical structures that consist of a reporter, a balancer, and a reactive group. Stable isotopes are incorporated at multiple positions so that the reporter region in each reagent of the set differs in mass, but the difference is compensated by a balancer, such that all tagged analytes (parent ion) have the same mass and appear as a single peak in the mass spectrum. However, after the parent ions are isolated and fragmented in a subsequent tandem mass spectrometry (MS/MS) measurement, a series of reporter ions are produced with differing masses, which allows for relative quantification of the parent ions or the relative abundance of the same analyte in different samples.
Isobaric tags offer two major advantages: 1) multiplex and high-throughput analysis of multiple samples; and 2) improved detection sensitivity because analytes from different samples contribute to the amplitude of a single peak. Two commercially available isobaric reagents are Tandem Mass Tag (TMT) from Thermo Scientific, and Isobaric Tag for Relative and Absolute Quantification (iTRAQ) from Applied Biosystems.
Despite the advantages of presently available isobaric tags, such as TMT and iTRAQ, these tags are limited to a total of 8 samples because the number of reporter ions are restricted by a limited structure having limited isotopic substitutions. These tags are also further limited by their high cost from tedious synthesis. Accordingly, isobaric tags having facile synthesis that allow for differential labeling of an unlimited number of samples are desired for quantifying analytes of interest.